In certain situations, it is desirable that a screw be both rotationally as well as axially releasably couplable with a driver. For instance, in limited-access work applications, e.g., in arthroscopic or other "closed" surgeries, where a screw is to be inserted into a hole which is not readily accessible, it is advantageous to axially couple the screw with the driver before inserting the screw into the surgical site. Such axial coupling is also beneficial where the screw implantation site is accessible, e.g. where the screw is inserted in a downwardly facing hole.
With such screw and driver combinations, the screw is first attached to the driver and then the driver is manipulated until the screw contacts the implantation site, whereupon the driver is rotated until the screw is fully implanted. Then, the driver is disengaged from the screw by pulling it axially away from the screw.
Such screw and driver combinations are known in the art, as illustrated by U.S. Pat. Nos. 2,329,398, 2,511,051, 2,775,913, 3,463,209, 3,695,321, and 3,888,144. Known screw and driver combinations typically suffer from one or more problems which limit their utility and acceptance in certain situations. For instance, known screw and driver assemblies when designed to provide sufficient axially locking engagement can be difficult to operate, inasmuch as the driver must be inserted with an unacceptably large axial force to overcome the frictional interference between screw and driver. With some screw and driver combinations, the rotational engagement between the screw and driver is not sufficiently positive, i.e., there often is an unacceptably large amount of play between the screw and driver. Other screw and driver combinations (1) tend to be relatively expensive to produce due to the relatively complex shapes of the screw and driver and (2) can permit some axial wobble between screw and driver, i.e., the combination is not designed to ensure the driver is always perfectly co-axially aligned with the screw.